Rekonstruktion und Relaxation metallischer Oberflächen

Wuttig, Matthias

Book/Report

FZJ-2018-02900

Rekonstruktion und Relaxation metallischer Oberflächen

Wuttig, M. (Corresponding author)FZJ*

1988
Kernforschungsanlage Jülich, Verlag Jülich

Jülich : Kernforschungsanlage Jülich, Verlag, Berichte der Kernforschungsanlage Jülich 2213, VI, 129 p. (1988)

Please use a persistent id in citations: http://hdl.handle.net/2128/18489

Report No.: Juel-2213

Abstract: High resolution electron energy loss spectroscopy has been used to study the relationship between surface structure and surface dynamics for different metal surfaces. The measurement of the Rayleigh wave dispersion for the clean Cu(100) surface shows a change of the force field in the vicinity of the surface. The experimental data can be explained either by an increase of the force constant k$_{12}$ between the first and the second layer or by an attractive (tensile) stress within the first layer. The increase in k$_{12}$ might be a consequence of an enlarged charge density between the first and second layer (relaxation), while the introduction of an attractive surface stress indicates a tendancy of the surface layer to contract as a consequence of an increased charge density within the first layer.With the concept of surface stress the frequency reduction of the Rayleigh wave for the c(2x2) oxygen overlayer can be explained at least phenomenological. In this case the surface stress is repulsive since the oxygen adsorption leads to a reduction of bonding charge between the first layer nickel atoms and hence a more effective repulsion of the ion cores. For carbon adsorption the charge transfer is even stronger and leads to a surface reconstruction in the lattice dynamical calculations as is observed by the experiment. These examples demonstrate the close connection between surface structure, surface dynamics and surface stress. Additionally they show the important role of vibrational spectroscopy, since this is one of the very few techniques for an experimental determination of microscopic surface stress. The measurement of the surface phonon dispersion for HfC(100) shows that the force field in the vicinity of the surface differs substantially from the bulk values. With a 20% increase of the force constant between first layer metal and second layer carbon atoms and a 40% reduction of the force constant between first layer carbon atoms and second layer metal atoms the experimental data can be reproduced fairly well. These bond changes indicate a structural relaxation where first layer carbon atoms are displaced outwards whereas first layer metal atoms are displaced inwards (rippled relaxation). Presumably the smoothing of the electron density is the driving force for the formation of this relaxation pattern. The investigation of the oxygen adsorption on Cu(100) shows a disordered adsorption for coverages below $\Theta_{o}$ =0.34. EELS spectra show that for these coverages two different adsorption sites are occupied. One of these adsorption sites should correspond to a local reconstruction where the first layer copper atoms show a contraction around the central oxygen atom. With increasing oxygen coverage a phase transition from the disordered (1x1) to the ordered ($\sqrt{2}$x2$\sqrt{2}$)R45 structure is observed at $\Theta_{o}$ = 0.34±0.02. This disorder-order transition can be followed by diffraction studies (diffuse elastic intensity and intensity of the superstructure reflexes) as well as vibration spectroscopy. The changes of the EELS spectra with increasing oxygen coverage and the appearance of a new mode for the ($\sqrt{2}$x2$\sqrt{2}$)R45 structure indicate an adsorbate induced reconstruction. This is a first order phase transition as can be seen for example by the intensity of the diffraction peaks. For a wellordered ($\sqrt{2}$x2$\sqrt{2}$)R45 structure the oxygen coverage was determined by an in-situ Auger calibration to 0.48±0.05. Vibrational spectra for this structure givefurther clues on the local adsorption geometry and allow to construct a reconstruction model. In this model second nearest neighbour rows in the [100]- direction show a row pairing. This reconstruction model is characterized by a symmertry reduction from C$_{4v}$ to C$_{2v}$ symmetry which might be the driving force for this reconstruction.

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